I think Lucy's wording "never bothered" is strong, but essentially correct. As I mentioned in a previous post, the reason he and Bonannini didn't finish editing DQ in 1970 was because Welles's affair with Olga Palinkas (as she was known at that time) was revealed in the scandal rags, and Welles decide to leave Italy in a huff. In Audrey Stainton's 1988 article on DQ all of this is detailed: how Welles entrusted DQ to Mauro, and then Beatrice picked it up, but later in 1974 Mauro's wife found that Welles had accidentally forgotten about it in an Italian vault, and the Bonaninni's had to save it as they could not locate Welles. Finally they did, and it was saved. This was 20 years before Welles died. I also have a tv interview in which Susan Cloutier describes how she also had an edit of DQ, and in a 1985 conversation with Welles she convinced him to pick up the project again. Welles thought it was a good idea, so he contacted Mauro and asked him to help him again with the editing; at this point, they had not worked on the picture for 15 years. Cloutier put the footage on a ship, and it was mid-Atlantic when she heard that Welles had died.
All of this leads to the inevitable conclusion that Welles just lost interest in it. Then the question begged is "why?". My guess is that, as with the book, there is was no dramatic arc to the story, and he couldn't figure out how to make it more than merely episodic; as a one hour tv program (which was the original plan, funded by ABC and Sinatra) it might have worked, but as a feature film, it would have been a much more difficult challenge. This is only a guess, of course, but the fact is that from 1970 to 1985 he did no work on DQ other than possibly a few landscape shots by Gary Graver in the early 70s. As Lucy says, if he had wanted to finish it, he certainly could have.